It is general that the printing surface of a printed material is rubbed in contact with such as guide rolls, turn bars, a former or the back surface of another printed material immediately after printing. The rubbing causes on the printed surface a so-called blocking phenomenon that the printed ink composition is fused to adhere to the surface of the contacted substance to considerably deteriorate the quality of the printed surface and also causes a problem to contaminate the contacted surface of the other substance.
To improve the abrasion resistance of the printed surface, microparticles of solid polymer such as resin or wax are added into the printing ink to form fine projections which provide the printed surface with smoothness.
The printing ink on the printed surface generally has a thickness of about 0.2 to 1.0 μm, so that the solid polymer particles for forming fine projections on the printed surface are adjusted to have a particle diameter in a range of 1 μm or more. It is because if the solid polymer particles have a particle diameter of less than 1 μm, the solid polymer particles are buried into the printed ink layer and do not act as a lubricant, and if the solid polymer particles have an excessively large particle diameter, the particles remain on the inking roll, the form plate or the blanket when printing to gradually accumulate or pile to deteriorate the printed image quality or aggregate on the form plate to form projections, and a so-called hickey phenomenon that the projected portions form non-print spots is caused.
But, it is hard to adjust the solid polymer particles to a desired grain size range and to disperse stably into an ink, and the problems of form plate stain, blanket stain and the like resulting from the inclusion of particles having a large particle diameter has not been solved completely yet.
When printing at a high speed, there is caused so-called misting that solid polymer particles are separated to get scattered when the ink is transferred from the form plate. This problem occurs because the solid polymer particles are low in affinity with an ink vehicle. To prevent it, it is necessary to modify the surfaces of the dispersed particles into the boundary surface having an affinity with the vehicle.
The high-speed offset printing exposes the printed material to a high temperature for drying, but if the paper surface temperature becomes 100° C. or more, the solid polymer particles projected from the surfaces are melted to make the surface flat, and the surface smoothness is lost. Therefore, the solid polymer particles having a high melting point are added to form fine projections on the printed surface in these years, but it is hard to make the average particle diameter of the solid polymer particles having a high melting point fine to about 1 to 10 μm.
In addition, slip properties between the printed surface and the back surface of the printed material and also its slip properties against the surface of another member are required, but the friction coefficient of the surface is rarely degraded considerably by wax or the like present as projections on the printed surface, and it becomes difficult to pile up the printed materials. In such a case, inorganic powder having an appropriate particle diameter is added into the printing ink to increase the friction coefficient, but there is a problem that the addition of the inorganic powder degrades the transparency and glossiness of the printed surface.